1. Field of the Invention
The present invention relates to a memory element in which recording of information can be performed and a memory device including the memory element.
2. Description of the Related Art
A DRAM operating at high-speed and having a high density is widely used as a random access memory in an information processing device such as a computer.
However, since a manufacturing process of the DRAM is complicated, manufacturing costs are high in comparison to a typical logic circuit LSI and signal processing which are used in an electronic device.
In addition, the DRAM is a volatile memory in which information disappears when the power is shut off, and it is necessary to perform frequently a refreshing operation, specifically, an operation in which written information (data) is read out, again amplified, and rewritten.
Then, an FeRAM (ferroelectric random access memory), MRAM (magnetic random access memory), and the like, for example are proposed as a nonvolatile memory in which information does not disappear even when the power is shut off.
In case of those memories, the written information can be retained for a long time even if the power is not supplied.
Further, in case of those memories, a refreshing operation becomes not necessary with the memory made nonvolatile, and it is assumed that the power consumption can be reduced to that extent.
However, in the above-described nonvolatile memory, it becomes difficult to secure a characteristic as a memory element due to a miniaturization of a memory element that constitutes each memory cell.
Therefore, it is difficult to miniaturize the element to the limit of a design rule and a limit on a manufacturing process.
Hereupon, a memory element of a new type has been proposed as a memory having a suitable structure for miniaturization.
This memory element has a structure in which an ion conductor containing a certain metal is positioned between two electrodes.
Further, since the metal contained in the ion conductor is made to be contained in one of the two electrodes, the metal contained in the electrode is diffused as an ion in the ion conductor when a voltage is applied between the two electrodes, and thereby an electric characteristic such as a resistance value or capacitance of the ion conductor changes.
A memory device can be made by using this characteristic (for example, refer to Patent reference 1 and Non-patent reference 1).
Specifically, the ion conductor is made of a solid solution of a chalcogenide and metal, further specifically, made of a material in which Cu, Ag, and Zn are solidly solved into AsS, GeS, and GeSe, and one of the two electrodes contains Cu, Ag, and Zn (refer to the-above described Patent reference 1).
Furthermore, various kinds of nonvolatile memories using a crystal oxide material have also been proposed, and a memory in a device, for example, having a structure in which a Cr-doped SrZrO3 crystal material is positioned between a lower electrode made of SrRuO3 or Pt and an upper electrode made of Au or Pt is reported, where a resistance is changed reversibly by applying a voltage of a different polarity (refer to the Non-patent reference 2). However, details of the principle and the like are not clear.
[Patent reference 1] Japanese Published Patent Application No. 2002-536840
[Non-patent reference 1] Nikkei electronics, Jan. 20, 2003 issue (page 104)
[Non-patent reference 2] A. Beck et al., Appl. Phys. Lett., 77, (year 2000), p. 139